In a typical transportation vehicle control system of the prior art, comma-free binary coded vehicle control information is transmitted to the transportation vehicles on a frequency modulated (FM), frequency shift key modulated (FSK), or phase shift key modulated (PSK) control signal. This signal is comprised of multiple message frequencies (e.g. 5KC and 10KC) which represent the binary logic conditions (1 and 0). These vehicle control signals have also been used to detect the presence of vehicles within a predetermined section of the vehicle pathway. A receiver compares logic conditions of the vehicle control signal transmitted from a location at one end of the predetermined section of the vehicle pathway with logic conditions detected from signals received at a location on the opposite end of the predetermined section of vehicle pathway. When the logic conditions which are detected do not correspond with the logic conditions of the vehicle control signal which is transmitted, it is presumed that a vehicle is present within the predetermined section of pathway. A problem with vehicle detection systems of the prior art has been that noise along the vehicle pathway, transient responses of the filters of the prior art receivers, and component failures in the prior art receivers, could result in the failure of the receiver to detect the presence of a vehicle, thereby creating an unsafe condition.
These detection failures of prior art receivers were due to the high gain of the limiting amplifier used in those receivers. If transient responses of the filters of the prior art receivers were sufficiently large, they were amplified to a limited maximum level by the limiting amplifier and erroneously detected as either a 1 or a 0 logic condition. If no vehicle control signal were received at the opposite end of the vehicle pathway, noise signals of relatively small amplitude could be amplified to a limited maximum level by the limiting amplifier and again erroneously detected as either a 1 or a 0 logic condition. If a filter of the prior art receiver suffered a failure, the limiting amplifier could also cause the receiver to detect erroneous logic conditions 1 or 0.
Because of the unsafe conditions created by prior art receivers which employed limiting amplifiers, there was a need for a vehicle detection receiver that could improve transportation system safety by eliminating the use of limiting amplifiers.